JP-A-11-153008 shows a VVT controller which is driven by an electric motor. In this VVT controller, a sun gear of a planetary gearing is rotated by the electric motor, thereby a phase between a ring gear and a carrier is varied. The planetary gearing functions as a phase converter. The ring gear is driven by a driving shaft of an engine and the carrier is connected with a cam shaft. The ring gear is referred to an input member and the carrier is referred to an output member in this invention.
In this kind of the VVT controller equipped with the electric motor, the motor shaft and the cam shaft rotate together with keeping a mechanical connection. When the motor shaft is locked for some reasons, the cam shaft is also locked. In this situation, an over-load applied to the phase converter and the cam shaft may cause damages of the phase converter, the engine, and a motor housing. When the motor housing is broken, an engine oil may flow out.